During spinal fusion surgery, bone attachment assemblies can be attached to bones. For example, bone screws, such as vertebral screws, may be fixed to adjacent vertebrae. Hooks or clamps may also be fixed to vertebra or ribs. The bone attachment assemblies that include bone screws, hooks, and/or clamps, may be interconnected to each other by a rod that attaches with multiple bone screws, hooks, and/or clamps in order to stabilize the spine during the healing process. The rods interconnecting the bone attachment assemblies may span one or more vertebral levels (e.g., one to three vertebral levels for a spinal fusion of a degenerative condition, eight to twelve vertebral levels for scoliosis, etc.).
In order to attach with a rod or connector, the bone attachment assemblies may include an attachment head with a slot that is intended to receive the rod. The rod may be at least partially secured within the slot by a cap or set screw.
Bone screws may be classified as monoaxial, polyaxial, or uniplanar based on their adjustability. Monoaxial bone attachment assemblies, such as monoaxial bone screws, are the most simplistic and are not particularly versatile in connection with accommodating screw placement and alignment. The attachment head of a monoaxial bone screw may allow the rod to be adjusted in only one direction, perpendicular to the longitudinal axis of the screw. When the rod is secured in the slot of the monoaxial bone screw, the longitudinal axis of the rod is substantially perpendicular (i.e., at approximately a 90° angle to) to the longitudinal axis of the monoaxial bone screw.
Polyaxial bone attachment assemblies, such as polyaxial screws, are commonly used to overcome the variations in screw placement and alignment. The relationship between the screw axis and the rod axis in a polyaxial screw may be adjustable but may still be locked in place (e.g., the attachment head of a polyaxial screw (and thus the rod) may be configured to swivel approximately 20° off the screw axis when unlocked). This adjustability may allow rods to be connected to multiple screws that may be placed medial or lateral to one another and may permit lordotic and kyphotic spinal alignments. Conventional polyaxial screws are typically larger and bulkier than monoaxial screws due to, for example, the structures that allow the attachment head of the polyaxial screw to swivel.
Uniplanar bone attachment assemblies, such as uniplanar screws, may have attachment heads that allow the rod be adjusted or moved within one plane relative to the screw axis. Typically, uniplanar screws do not allow the rod to be adjusted to medial or lateral rod positions (unlike polyaxial screws). Uniplanar screws are more commonly used in scoliosis surgery where there may be a degree of cranial or caudal angulation (such as the sagittal plane of the spine), but where there is little medial lateral screw placement deviation and the surgeon additionally needs rigid control of the screw to manipulate it in the coronal and axial plane of the spine. Similar to polyaxial screws, uniplanar screws generally are large and bulky due to the configuration of their single plane swivel mechanism.
The connecting rods that can attach with and connect to multiple bone attachment assemblies, such as bone screws, experience significant loads from the secured bone attachment assemblies attached to the patient for correction. Such loads may cause the connecting rods to slip within the bone attachment assemblies.
Monoaxial bone screws, which are designed to receive and secure a connecting rod at a fixed angle (such as 90 degrees) with respect to the screw shaft, may include a recess formed in the attachment head of the monoaxial bone screw to permit increased loads to be applied to a connecting rods disposed therein. However the rigid structure of such monoaxial bone screws disadvantageously constrains the mobility of a connecting rod held therein, such that movement of the rod away from the fixed angle eliminates contact points between the monoaxial bone screw and the connecting rod, thereby increasing the risk of rod slippage.